1. Field
The present disclosure relates generally to aircraft and, in particular, to a method and apparatus for an aircraft joint. Still more particularly, the present disclosure relates to a method and apparatus for an aircraft joint with improved airflow.
2. Background
A fixed-wing aircraft may be a heavier-than-air vehicle capable of flying in the air. A fixed-wing aircraft may be capable of flying due to lift, which may be generated by the wings of the aircraft. Lift may be a component of an air dynamic force acting on the aircraft. This force may be perpendicular to an oncoming flow direction. During flight, an aircraft may also experience a drag force that may be in the direction of air flow.
The performance of an aircraft may be improved through the reduction of drag that may occur when air flows over the surface of an aircraft. Laminar flow may be used to reduce drag on an aircraft. Laminar flow occurs when air flows in parallel layers without mixing between the layers of air.
With an aircraft wing, air flowing over the wing may have a boundary layer. A boundary layer may be a very thin layer of air lying over the surface of the wing and other surfaces of the aircraft. This layer of air may tend to adhere to the aircraft wing. As the aircraft wing moves forward through the air, the boundary layer at first may flow smoothly with laminar flow over the wing.
Surface discontinuities may disrupt the laminar boundary layer of air over an aircraft wing and cause it to become turbulent. A turbulent boundary layer may be characterized by increased mixing between layers of air within the boundary layer. The drag caused by a turbulent boundary layer may be as much as around 5 to 10 times larger than the drag of a laminar boundary layer. This transition from laminar flow to turbulent flow within the boundary layer may increase drag. Also, this transition may produce undesirable noise, decrease fuel efficiency, and/or other undesirable effects during flight. These discontinuities may be caused by, for example, without limitation, steps and/or gaps in joints between skin panels on an aircraft.
Maintaining a laminar boundary layer past joints between skin panels on a painted aircraft may require changes in the surface that may have tight tolerances to avoid turbulent flow. For example, these changes may include tolerances for a step in the surface, a gap, waviness, and/or other discontinuities on the surface of the aircraft. A step may be a deviation from a plane across a skin panel joint on a surface. A step also may be an abrupt deviation from a desired smoothness for the surface.
For example, a tolerance for a step may be around 0.002 inches or less to maintain a laminar boundary layer past a joint. Gaps within this tolerance may be from around 0.020 inches to around 0.050 inches. This tolerance may be difficult to achieve at joints in which skin panels meet each other with currently used manufacturing and/or installation methods for skin panels for aircraft. Further, the removal and/or reinstallation of skin panels during maintenance also may provide challenges to meeting desired tolerances for maintaining laminar flow over the surface of an aircraft.
One solution involves filling a gap between skin panels with a filler such as, for example, without limitation, a hardened epoxy filler putty. With this type of solution, fastener heads may be recessed below the surface. These fastener heads may then be covered with an epoxy filler. The epoxy filler may then be sanded to obtain the desired smoothness with respect to the surface of the skin panels. Further, mismatches at low spots at which the skin panels are attached to the support structure also may be filled with an epoxy filler and sanded down.
This type of approach, however, may be labor intensive and increase the cost of manufacturing or maintaining an aircraft. Further, the durability of these types of fillers may require reapplication of the fillers more frequently than desired. As a result, an aircraft may be out of service for maintenance more often than desired.
Therefore, it would be advantageous to have a method and apparatus that takes into account one or more of the issues discussed above, as well as possibly other issues.